1. Field of the Invention
The present invention refers to optical communication lines which use waveguide connections, in particular, optical fiber connections. More in detail, this invention refers to dispersion compensation maps implemented by optical communication lines.
2. Description of the Related Art
Conventional optical communication systems use optical fibers which extend from a transmitting to a receiving station with repeating or amplifying stations placed at intervals comprising, for example, optical fiber amplifiers
The amplifying stations amplify the strength of the optical signal they receive to enable propagation along the portion of fiber (commonly called “span”) which separates the station from a subsequent amplifying station or from the receiving station.
During propagation, the optical signals may be subjected to distortion associated with non linear effects. In fact, said non linear effects may be of a considerable magnitude since they are directly correlated to the strength of the optical signal kept, typically, at high levels by means of the amplifying stations.
In the case of long distance digital transmission (for example, in the order of hundreds of km) such as, for example, transmissions which use return to zero RZ or non-return to zero NRZ modulation, this type of distortion can be particularly detrimental.
Among the non linear effects which impair optical fiber transmission, the intrachannel non linear effect is particularly detrimental. In particular, said intrachannel non linear effects are the intrachannel cross-phase modulation IXPM and the effect known as intrachannel four-wave mixing IFWM. These effects, together with the Kerr effect, lead to a distortion of the pulse transmitted on the optical fiber as a result of interaction with pulses transmitted on the same carrier wavelength and, therefore, belonging to the same channel.
Techniques to reduce or compensate non linear effects (among which the intrachannel effect) by means of appropriate chromatic dispersion maps are already known. The communication systems which use said maps are known as Dispersion Managed Systems.
The article by Shiva Kumar et al. “Intrachannel Nonlinear Penalties in Dispersion-Managed Transmission Systems” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 8, No. 3, May/June 2002, describes a mathematical model of intrachannel non linear effects. The authors of this article propose, in order to reduce said intrachannel effects, the use of a dispersion map where dispersion compensation within each span is only partial, and the residual dispersion is compensated by means of the introduction of pre-compensation and post-compensation fibers, placed at the beginning and the end of an optical connection.
The article by M. Zitelli et al. “Single-Channel Transmission in Dispersion Management Links in Conditions of Very Strong Pulse Broadening: Applications to 40 Gb/s signals on step-Index Fibers” Journal of Lightwave Technology, Vol. 17, No. 12, December 1999, describes an analysis carried out on a Dispersion Managed System for single channel transmissions. In order to control propagation of the single pulse and to reduce nonlinear pulse interaction, the introduction of a prechirp is proposed, i.e. induction of dispersion in the signal to be introduced at the beginning of the span of an optical line compensated in chromatic dispersion.
The article by A. Mecozzi et al. “Analysis of Intrachannel Nonlinear Effects in Highly Dispersed optical Pulse Transmission”, IEEE Photonics Technology Letters, Vol. 12, No. 4, April 2000, pages 392-394, analyzes the intrachannel nonlinear effects in high bit rate transmission systems, considering short optical pulses which are dispersion compensated and propagated in optical fibers. The authors, also quoting other studies on the subject, affirm that the magnitude of nonlinear effects can be reduced monotonically by reducing the pulse width and increasing the dispersion coefficient. In particular, this article shows how, by dispersing the pulses rapidly along the fiber, it is possible to reproduce, after an appropriate dispersion compensation, the original pulse sequence which is only slightly affected by the nonlinear effects of the fiber.
In order to further clarify the description which follows, some definitions concerning chromatic dispersion are given hereunder.
First order chromatic dispersion β2 (hereinafter called “chromatic dispersion”) is given by the following formula:
      β    2    =                    ⅆ        2                    ⅆ                  ω          2                      ⁢    β  
corresponding to the second derivative of the propagation constant β compared to the angular frequency ω.
Dispersion in optical fiber of radiation with wavelength λ is also indicated with the dispersion parameter D given by the expression:
  D  =      -                  2        ⁢        π        ⁢                                  ⁢                  β          2                            λ        2            
Another parameter which characterizes the dispersion behavior of a fiber is the chromatic dispersion slope S, linked to the second order dispersion β3 (equal to the third derivative of the propagation constant β).
Furthermore, it is pointed out that considering two connected portions of optical fibers having lengths of L1 and L2 with dispersion parameters of D1 and D2 respectively, the accumulated dispersion Dacc on the optical path L1+L2 is defined as follows:Dacc=D1L1+D2L2
The patent application EP-A-1263155 affirms that one of the largest factors limiting optical transmission of data in WDM systems (Wavelength Division Multiplexing) is the effect of the chromatic dispersion slope S, which is a function of the wavelength. Said patent application describes various optical transmission lines and various line portions where compensation of the dispersion D and of the dispersion slope S is carried out. In particular, transmission lines, composed of several fiber portions and configured according to various dispersion maps, are described. For the production of a transmission line portion, it is suggested to choose the length, the dispersion parameter D and the dispersion slope S of the fibers which make up the line portion in such a way that the average <S> of the dispersion slope of the optical path is zero and in such a way that the average <D> of the dispersion is different to zero. This document shows how by reducing to zero the average <S> of the dispersion slope makes these transmission lines particularly interesting for WDM systems since uniform dispersion behavior for the various channels is ensured. Among the various maps described in this document, an optical transmission line portion is presented which comprises two spans 10, each including a first fiber 12 (leaving an amplifier 16) connected to a second fiber 14 connected to the input of another amplifier 16. The fiber 12 is single mode and has a dispersion parameter equal to 16.2 ps/nm/Km, estimated at a wavelength of 1550 nm. The fiber 14 is an RDF fiber (Reverse Dispersion Fiber) which has a dispersion of −15.36 ps/nm/Km, estimated at a wavelength of 1550 nm.
Furthermore, FIG. 11 of the patent application EP-A-1263155 illustrates spans formed by the fibers 12 and 14, arranged in such a way that the fiber 12 (with positive dispersion equal to 16.2 ps/nm/Km) and the fiber 14 with negative dispersion equal to −20 ps/nm/km) are connected to the output of two successive amplifiers 16.
The Applicant observes that in the above-mentioned patent application EP-A-1263155, no mention is made of possible dispersion map configurations which could limit nonlinear effects.
The Applicant has faced the problem of supplying an optical communication line where the nonlinear effects and, in particular, the intrachannel nonlinear effects are limited.
The Applicant observes that the links existing between intrachannel nonlinear effects and dispersion are still not entirely clear and cannot be described mathematically with complete precision. This makes synthesis of Dispersion Managed Systems having a satisfactory limitation of nonlinear effects particularly difficult.